Adjusting nitrogen oxide ratios in exhaust gas

ABSTRACT

A method of adjusting a ratio of NO to NO 2  in a flow of exhaust gas is disclosed. The method includes directing a first portion of the exhaust flow through at least one first channel, wherein the at least one first channel is a wall-flow channel, and directing a second portion of the exhaust flow through at least one second channel, wherein the at least one second channel is an open flow-through channel. The method further includes converting NO into NO 2  at a first rate in the first portion of the exhaust flow as the first portion of the exhaust flow passes through the at least one first channel and converting NO into NO 2  at a second rate in the second portion of the exhaust flow as the second portion of the exhaust flow passes through the at least one second channel, the second rate being slower than the first rate.

TECHNICAL FIELD

The present disclosure is directed to treating exhaust gas expelled by internal combustion engines and, more particularly, to a system and method for adjusting nitrogen oxide ratios in exhaust gas.

BACKGROUND

Internal combustion engines, including diesel engines, gasoline engines, gaseous fuel-powered engines, and other engines known in the art, exhaust a complex mixture of air pollutants. These air pollutants may include solid material, known as particulate matter or soot, and gaseous compounds, such as hydrocarbons and the oxides of nitrogen (NO_(X)). Due to increased concern over environmental pollution, exhaust emission standards have become more stringent and the amount of particulate matter and NO_(X) emitted from an engine may be regulated depending on the type of engine, size of engine, and/or class of engine.

One method implemented by engine manufacturers for complying with the regulation of particulate matter exhausted to the environment has been to remove the particulate matter from the exhaust flow by using a device called a diesel particulate filter (“DPF”). A DPF typically consists of a wire mesh or ceramic honeycomb medium which is designed to trap particulate matter. Trapped particulate matter may be then be burned away using passive or active regeneration methods. In some cases, a DPF may be coated with a catalyst so as to form a coated diesel particulate filter (“CDPF”). The coating of catalyst may be used to convert hydrocarbons, carbon monoxide, and/or nitrogen oxides in the exhaust flow into less noxious products.

In addition to using DPFs and/or CDPFs for particulate filtration and conversion, some engine manufacturers have also implemented a process called Selective Catalytic Reduction (“SCR”) in order to reduce NO_(X) from exhaust gas. SCR is a process in which gaseous or liquid reductant (most commonly a solution of ammonia or urea solid and water) is added to the exhaust gas stream of an engine and is adsorbed onto a catalyst. The reductant reacts with NO_(X) in the exhaust gas to form H₂O and N₂, which can be safely released into the atmosphere.

Although SCR can be an effective method for reducing NO_(X), the efficiency of the process may be dependent on the concentrations of hydrocarbons, CO, NO, and NO₂ in the exhaust gas, before it is reacted with the SCR reductant. For example, the SCR process may be performed more efficiently when the ratio of NO to NO₂ is approximately 1:1 (i.e., 50% NO and 50% NO₂). This condition may be difficult to achieve because exhaust gas typically contains a ratio of NO to NO₂ of approximately 10:1 (i.e., 90-95% NO and 5-10% NO₂), when it exits a combustion engine. Therefore, it may be desirable to convert a portion of NO into NO₂, for example, by using a CDPF that has been coated with a catalyst selected to oxidize NO into NO₂. The CDPF may be positioned upstream from an SCR catalyst in order to selectively optimize the ratio of NO to NO₂, and also reduce particulates and hydrocarbons in the exhaust gas, before it undergoes the SCR process. Even when such a CDPF is used to bring the ratio of NO to NO₂ somewhat closer to 1:1 prior to SCR, the method may be ineffective and/or inefficient. Specifically, if the CDPF is coated with a catalyst that is ideal for NO oxidation, the CDPF will likely create an abundance of NO₂, thereby resulting in a ratio of NO to NO₂ which is even lower than the desired 1:1 ratio. Moreover, a CDPF coated with such a catalyst may be unable to trap particulate matter and/or convert hydrocarbons, as desired before the SCR process.

One way to remedy this problem is to divide the exhaust flow into two separate gas streams and treat the NO of each gas stream in a different manner. An example of a system that implements this method is described in U.S. Pat. No. 6,846,464 (the '464 patent) issued to Montreuil et al. on Jan. 25, 2005. Specifically, the '464 patent discloses a system for reducing NO_(X) by sending a first gas stream through a catalytic chamber coated with platinum, and sending a second gas stream through a catalytic chamber coated with palladium. The platinum-coated chamber oxidizes hydrocarbons and oxidizes the NO into NO₂, while the palladium-coated chamber oxidizes only hydrocarbons and leaves the NO essentially unreacted. The two gas streams are then recombined to form a gas flow, which is approximately a 50:50 mixture of NO/NO₂, before it is sent to an SCR catalyst. The '464 patent also discloses that the platinum-coated chamber and palladium-coated chamber may both be flow-through-type monoliths, or they may both be wall-flow-type particulate filters.

Although the system of the '464 patent may bring the ratio of NO and NO₂ somewhat closer to the desired ratio of 1:1, it may be insufficient for producing the particular composition of exhaust flow which is desired to be sent to the SCR catalyst. Moreover, the system may result in inefficient engine operation. Specifically, if both the platinum-coated chamber and palladium-coated chamber are flow-through-type monoliths, the system may be unable to trap enough particulate matter to meet emissions standards. Moreover, if both the platinum-coated chamber and palladium-coated chamber are wall-flow-type particulate filters, soot may build up in the palladium-coated chamber, thereby causing back pressure in the exhaust system and inefficient operation of the combustion engine.

The system and method of the present disclosure solve one or more of the problems set forth above.

SUMMARY OF THE INVENTION

One aspect of the present disclosure is directed to a method of adjusting a ratio of NO to NO₂ in a flow of exhaust gas. The method may include directing a first portion of the exhaust flow through at least one first channel, wherein the at least one first channel is a wall-flow channel, and directing a second portion of the exhaust flow through at least one second channel, wherein the at least one second channel is an open flow-through channel. The method may further include converting NO into NO₂ at a first rate in the first portion of the exhaust flow as the first portion of the exhaust flow passes through the at least one first channel and converting NO into NO₂ at a second rate in the second portion of the exhaust flow as the second portion of the exhaust flow passes through the at least one second channel, the second rate being slower than the first rate. The method may further include removing particulate matter from the first portion of the exhaust flow as the first portion of the exhaust flow passes through the at least one first channel.

Yet another aspect of the present disclosure is directed to a method of altering a composition of exhaust gas flow including particulate matter, hydrocarbons, and NO. The method may include directing a first portion of the exhaust flow through at least one first channel, wherein the at least one first channel is a wall-flow channel, and directing a second portion of the exhaust flow through at least one second channel, wherein the at least one second channel is an open flow-through channel. The method may further include converting NO into NO₂ in the first portion of the exhaust flow as the first portion of the exhaust flow passes through the at least one first channel, and trapping the particulate matter in the first portion of the exhaust flow as the first portion of the exhaust flow passes through the at least one first channel. The method may further include passively regenerating the particulate matter in the second portion of exhaust flow as the second portion of exhaust flow passes from a first end of the at least one second flow channel through a second end of the at least one second flow channel.

Yet another aspect of the present disclosure is directed to an exhaust filter. The exhaust filter may include at least one first channel configured to convert NO in an exhaust flow to NO₂ at a first rate, the at least one first channel being a wall-flow channel configured to remove particulate matter from the exhaust flow. The exhaust filter may further include at least one second channel configured to convert NO in an exhaust flow to NO₂ at a second rate, slower than the first rate, wherein the at least one second channel is an open flow-through channel.

Yet another aspect of the present disclosure is directed to a method of adjusting a ratio of NO to NO₂ in a flow of exhaust gas. The method may include directing a first portion of the exhaust flow through at least one first channel, wherein the at least one first channel is a wall-flow channel, and directing a second portion of the exhaust flow through at least one second channel, wherein the at least one second channel is an open flow-through channel. The method may further include transforming NO into NO₂ in the first portion of the exhaust flow to yield a first ratio of NO to NO₂ and transforming NO into NO₂ in the second portion of the exhaust flow to yield a second ratio of NO to NO₂. The method may further include combining the first and second portions of the exhaust flow as the first and second portions of the exhaust flow exit the at least one first and second flow channels to yield a final ratio of NO to NO₂ of about 1:1 in the combined exhaust flow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic illustration of an exemplary disclosed power system and exhaust treatment system;

FIG. 2 is a perspective view of a cut-away portion of an exemplary disclosed wall-flow particulate filter;

FIG. 3 is a perspective view of a cut-away portion of an exemplary disclosed flow-through substrate;

FIG. 4 is a diagrammatic illustration of an exemplary disclosed hybrid particulate filter; and

FIG. 5 is a diagrammatic illustration of an alternative exemplary disclosed hybrid particulate filter.

DETAILED DESCRIPTION

FIG. 1 illustrates a power system including a power unit 10 and an exhaust treatment system 12. In one embodiment, the power system may be associated with a mobile vehicle such as a passenger vehicle, a vocational vehicle, a farming vehicle or a construction vehicle. Alternatively, the power system may be associated with a stationary machine such as an industrial power generator or a furnace.

For the purposes of this disclosure, power unit 10 is depicted and described as a four-stroke diesel engine having an engine block 14, which at least partially defines four combustion chambers. One skilled in the art will recognize, however, that power unit 10 may be any other type of internal combustion engine such as, for example, a gasoline engine, a gaseous fuel-powered engine, or a turbine engine. Moreover, it is contemplated that power unit 10 may include a greater or lesser number of combustion chambers and that the combustion chambers may be disposed in an “in-line” configuration, a “V” configuration, or any other suitable configuration.

Exhaust treatment system 12 may include an exhaust manifold 20 having a plurality of exhaust passageways, each exhaust passageway being in fluid communication with an associated one of the combustion chambers of power unit 10. Exhaust manifold 20 may be configured to expel exhaust flow away from the combustion chambers of power unit 10 and towards a first exhaust channel 22 and a second exhaust channel 24. Moreover, exhaust manifold 20 may be configured to divide the flow of exhaust leaving power unit 10 between first exhaust channel 22 and second exhaust channel 24, at a certain proportion of exhaust flow. In one embodiment, exhaust manifold 20 may be integrally formed with first and second exhaust channels 22, 24. First and second exhaust channels 22, 24 may be cylindrical or tubular conduits configured to direct exhaust gasses and particulates away from power unit 10 for processing by various emission controlling devices. Therefore, in one embodiment, each of first and second exhaust channels 22, 24 may be provided with one or more emission controlling devices of exhaust treatment system 12.

The one or more emission controlling devices of exhaust treatment system 12 may include various particulate filters and NO_(X) aftertreatment devices. Particulate filters and NO_(X) aftertreatment devices may be disposed across the cylindrical width (i.e., cross section) of first and second exhaust channels 22, 24. Furthermore, the particulate filters and NO_(X) aftertreatment devices may be either removably or fixedly secured at their respective perimeters to first and second exhaust channels 22, 24. Exhaust treatment system 12 may also include devices (not shown) that provide a supply of catalyst solution to a fuel and/or the exhaust flow at any location along exhaust treatment system 12. The exhaust catalyst or fuel born catalyst (“FBC”), may include, for example, platinum, palladium, copper, cerium, manganese and/or iron compounds. Exemplary commercial catalysts contemplated for use include Eolys®, which is marketed by Rhodia; Platinum Plus®, which is marketed by Clean Diesel Technologies; Octimax 4800®, which is marketed by Octel; and, MMT, which is marketed by Ethyl Corporation. It is contemplated that exhaust treatment system 12 may include other components such as, for example, a turbine, an exhaust gas recirculation system, a catalytic treatment device, or any other exhaust system component known in the art.

In one embodiment, first exhaust channel 22 may be provided with a particulate filter 26, and second exhaust channel 24 may be provided with a flow-through substrate 28. Particulate filter 26 may include any variety of diesel particulate filter (“DPF”) such as, for example, a corderite or silicon carbide wall-flow filter and/or a metal fiber partial filter. In one exemplary embodiment, particulate filter 26 may be a wall-flow particulate filter configured to trap diesel particulates at a desired rate. Moreover, particulate filter 26 may be a coated diesel particulate filter (“CDPF”), which includes a coating of one or more catalysts configured to reduce noxious components of the exhaust flow. For example, particulate filter 26 may be a CDPF coated with a platinum (Pt) catalyst configured to convert NO to NO₂ at a first rate. Flow-through substrate 28 may be a metal fiber flow-through filter. In one embodiment, flow-through substrate 28 may be coated with a palladium (Pd) catalyst which is configured to convert NO into NO₂ at a second rate. Thus, exhaust treatment system 12 may include a particulate filter 26, which is a wall-flow type CDPF having a platinum coating, and a flow-through substrate 28, which is coated with a palladium catalyst.

FIG. 2 depicts the function of an exemplary wall-flow particulate filter 30. Wall-flow particulate filter 30 may include a plurality of first and second blocked passageways 32, 34 formed by an assembly of porous filter walls 31 and flow blocks 36. Flow blocks 36 may be disposed at alternating, opposite ends of adjacent first and second blocked passageways 32, 34. For example, as depicted in the portion of wall-flow particulate filter 30 illustrated in FIG. 2, each second blocked passageway 34 may have a flow block 36 provided adjacent to an exhaust entry 33, whereas a first blocked passageway 32 disposed between second blocked passageways 34 may have a flow block 36 provided adjacent to an exhaust exit 35. Porous filter walls 31 may be configured to trap particulates while allowing exhaust gases to pass into adjacent blocked passageways having unblocked exhaust exits 35. For example, porous filter walls 31 may be made from wire mesh, ceramic honeycomb, or any other material known in the art which is suitable for trapping exhaust particulates. Thus, exhaust flow entering first blocked passageways 32 via exhaust entries 33 (solid flow arrows) may be restricted by flow blocks 36, and forced into adjacent second blocked passageways 34 through porous filter walls 31, after which the exhaust flow may exit wall-flow particulate filter 30 via exhaust exits 35 (dashed flow arrows). As described above with respect to FIG. 1, porous filter walls 31 of wall-flow particulate filter 30 may be coated with a platinum catalyst selected to convert NO into NO₂ at a first conversion rate.

FIG. 3 depicts the function of an exemplary flow-through substrate 40. Flow-through substrate 40 may include a plurality of open exhaust passageways 42 created by a lattice of porous filter walls 44. Porous filter walls 44 may be configured to trap particulates contained in exhaust gas flowing from exhaust entries 46 to exhaust exits 48 along porous filter walls 44 of each open exhaust passageway 42. For example, porous filter walls 44 may be made from wire mesh, ceramic honeycomb, or any other material known in the art which is suitable for trapping exhaust particulates. In one embodiment, flow-through substrate 40 may trap particulates in porous filter walls 44 at a lower rate than the rate at which particulate matter is trapped in porous filter walls 31 of wall-flow particulate filter 30. As described above with respect to FIG. 1, porous filter walls 44 of flow-through substrate 40 may be coated with a palladium catalyst selected to convert NO into NO₂ at a second conversion rate, which may be lower than the first conversion rate of wall-flow particulate filter 30. Although FIG. 1 depicts particulate filter 26 and flow-through substrate 28 as being separate devices, it is contemplated that both wall-flow and flow-through type passageways may be combined into any type of single hybrid particulate filter.

FIG. 4 depicts an embodiment of particulate filter 26 and flow-through substrate 28 provided together in a hybrid particulate filter 50. Hybrid particulate filter 50 may include a plurality of first exhaust passageways 56 which form a wall-flow type particulate filter and a plurality of second exhaust passageways 54 which form a flow-through substrate. First exhaust passageways 56 may have flow blocks 58 at alternating, opposite ends so as to force exhaust flow to migrate through walls between adjacent first exhaust passageways 56, as described above with respect to wall-flow particulate filter 30. Second exhaust passageways 54 may be open flow-through type passageways which allow exhaust flow to travel freely from a first end 52 to a second end 59 of hybrid particulate filter 50. Thus, exhaust flow entering hybrid particulate filter 50 at a first end 52 may travel to second end 59 via both first exhaust passageways 56 and second exhaust passageways 54. In one embodiment, first exhaust passageways 56 may be coated with a platinum catalyst and second exhaust passageways 54 may be coated with a palladium catalyst.

FIG. 5 depicts an alternative embodiment of a hybrid particulate filter 60. Hybrid particulate filter 60 may include a plurality of first exhaust passageways 63 which form a wall-flow type particulate filter and a plurality of second exhaust passageways 64 which form a flow-through type substrate. First exhaust passageways 63 may have flow blocks 66 at alternating, opposite ends so as to force exhaust flow to migrate through walls between adjacent first exhaust passageways 63, as described above with respect to wall-flow particulate filter 30. Second exhaust passageways 64 may be open flow-through type passageways which allow exhaust flow to travel freely from a first end 62 to a second end 68 of hybrid particulate filter 60. Thus, exhaust flow entering hybrid particulate filter 60 at first end 62 may travel to second end 68 via both first exhaust passageways 63 and second exhaust passageways 64. In one embodiment, first exhaust passageways 63 may be coated with a platinum catalyst and second exhaust passageways 64 may be coated with a palladium catalyst. As illustrated in the embodiment of FIG. 5, second exhaust passageways 64 may be disposed closer to a central axis of hybrid particulate filter 60. First exhaust passageways 63 may be disposed around and outside of such a central core of second exhaust passageways 64 in hybrid particulate filter 60. When open flow-through type, second exhaust passageways 64 are aligned with a center of exhaust flow, they may promote more uninhibited flow of exhaust through hybrid particulate filter 60.

In one embodiment, the wall-flow type passages of first particulate filter 26 or hybrid particulate filters 50, 60 may be coated with a platinum catalyst, whereas the flow-through type passages of flow-through substrate 28 or hybrid particulate filters 50, 60 may not be coated with any catalyst at all. In another embodiment, the flow-through type passages may be coated with both platinum and palladium catalysts. In yet another embodiment, the flow-through type passages may be coated only with a palladium catalyst. The flow-through type passages may alternatively be coated with any combination of base metal catalysts, such as nickel, cobalt, copper, chromium, iron, and zinc based catalysts. In one embodiment, exhaust flow from power unit 10 may be evenly distributed between wall-flow and flow-through passageways (i.e., about 50% to 50%) of either particulate filter 26, flow-through substrate 28, and/or one of hybrid particulate filters 50, 60. Alternatively, about 70-95% of the exhaust flow may be directed through wall-flow type passages, while about 5-30% of the exhaust flow may be directed through flow-through type passages.

INDUSTRIAL APPLICABILITY

The disclosed exhaust treatment system of the present disclosure may be applicable to any combustion-type device such as, for example, an engine, a furnace, or any other device known in the art wherein it is desirable to remove particulate pollutants from an exhaust flow. The disclosed exhaust treatment system may be a simple, inexpensive, and effective solution for adjusting nitrogen oxide ratios while removing particulates from an exhaust flow. The operation of power unit 10 and exhaust treatment system 12 will now be explained.

Referring to FIG. 1, fuel may be injected into combustion chambers of power unit 10 by fuel injectors, mixed with the air therein, and combusted by power unit 10 to produce a mechanical work output and an exhaust flow of hot gases. The exhaust flow may contain a complex mixture of air pollutants, which can include noxious gasses and solid particulates such as soot. As exhaust gases are expelled from power unit 10 into exhaust manifold 20, the exhaust gases may be split between first exhaust channel 22 and second exhaust channel 24. Exhaust gases flowing through first and second exhaust channels 22, 24 may then be treated by particulate filter 26 and flow-through substrate 28, respectively, before being released into the environment. Alternatively, exhaust gases expelled from power unit 10 may be directed through one of hybrid particulate filters 50, 60. As described above, exhaust flow from power unit 10 may be evenly distributed between wall-flow and flow-through passageways (i.e., about 50% to 50%) of either particulate filter 26 and flow-through substrate 28, or one of hybrid particulate filters 50, 60. Alternatively, about 70-95% of the exhaust flow may be directed through wall-flow type passages, while about 5-30% of the exhaust flow may be directed through flow-through type passages. Thus, exhaust gases expelled by power unit 10 may be treated before being released into the environment.

Specifically, the release of soot into the environment may be reduced by passing the exhaust flow through particulate filter 26 and flow-through substrate 28, or one of hybrid particulate filters 50, 60. As this soot laden exhaust flow is directed from the combustion chambers through particulate filter 26, soot may build up in particulate filter 26. When the temperature of the exhaust flow is higher than the oxidation temperature of the soot, the soot may slowly “burn off” of particulate filter 26 over time, thereby passively regenerating particulate filter 26. Because exhaust treatment system 12 may include at least some amount of wall-flow type particulate filtration (i.e., using particulate filter 26 or wall-flow portions of one of hybrid particulate filters 50, 60), the amount of particulates released into the air may be reduced in comparison to the particulate pollution that would occur using only flow-through type filtration. However, because exhaust treatment system 12 may also include at least some amount of flow-through type exhaust treatment (i.e., using flow-through substrate 28 or flow-through portions of one of hybrid particulate filters 50, 60), the amount of exhaust back-pressure experienced using purely wall-flow filtration may be reduced. Thus, the performance of power unit 10 may be improved in comparison to the performance that would occur if all of the exhaust flow were directed through wall-flow filtration. Moreover, because the wall-flow passageways are known to trap larger amounts of particulates than flow-through passageways, the coating of platinum catalyst on the wall-flow passageways may advantageously provide a higher rate of passive regeneration of those wall-flow passageways. Therefore, the coating of platinum catalyst may improve the effectiveness and durability of the wall-flow portions of the particulate filters.

As described above, the exhaust flow expelled by power unit 10 may initially include a ratio of NO to NO₂ of about 95% NO to 5% NO₂. In one embodiment, platinum-coated filter portions of exhaust treatment system 12 may convert NO to NO₂ at a faster rate than palladium-coated filter portions of exhaust treatment system 12. Because exhaust treatment system 12 may pass at least some of the exhaust flow through a particulate filter coated with a platinum catalyst, there may be a conversion rate of oxidation of NO into NO₂, which is high enough to obtain a ratio of NO to NO₂ of at least about 50% NO to 50% NO₂. However, because exhaust treatment system 12 also passes at least some of the exhaust flow through a flow-through substrate that is either: not coated with a catalyst; coated with platinum and palladium catalysts; or coated with a palladium catalyst, the conversion rate of oxidation of NO into NO₂ may be low enough to avoid causing a ratio of NO to NO₂, which is substantially lower than 50% NO to 50% NO₂. Thus, the above described system and method may provide advantages over those prior systems using only a platinum catalyst, which has been known to cause a concentration of NO₂ as high as about 90% NO₂. Specifically, the above described embodiments may be useful for creating a composition of exhaust gases including a final ratio of NO to NO₂ of about 50% NO to 50% NO₂, which may be ideal for treatment by a NO_(X) aftertreatment device, such an SCR catalyst. Such an even ratio of NO to NO₂ may allow a downstream NO_(X) aftertreatment device, such an SCR catalyst, to achieve nearly 100% conversion of nitrogen oxides into pure nitrogen gas (N₂), which may be safely released into the atmosphere.

It will be apparent to those skilled in the art that various modifications and variations can be made to the system of the present disclosure without departing from the scope of the disclosure. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the system disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope of the disclosure being indicated by the following claims and their equivalents. 

1. A method of adjusting a ratio of NO to NO₂ in a flow of exhaust gas, the method comprising: directing a first portion of the exhaust flow through at least one first channel, wherein the at least one first channel is a wall-flow channel; directing a second portion of the exhaust flow through at least one second channel, wherein the at least one second channel is an open flow-through channel; converting NO into NO₂ at a first rate in the first portion of the exhaust flow as the first portion of the exhaust flow passes through the at least one first channel; converting NO into NO₂ at a second rate in the second portion of the exhaust flow as the second portion of the exhaust flow passes through the at least one second channel, the second rate being slower than the first rate; and removing particulate matter from the first portion of the exhaust flow as the first portion of the exhaust flow passes through the at least one first channel.
 2. The method of claim 1, wherein the first and second rates are selected to provide a final ratio of NO to NO₂ of about 1:1 in the exhaust flow.
 3. The method of claim 1, further comprising converting hydrocarbons in one or both of the first and second portions of the exhaust flow as the first and second portions of the exhaust flow pass through the at least one first channel and the at least one second channel, respectively.
 4. The method of claim 1, wherein NO is converted into NO₂ at the first rate in the first portion of the exhaust flow using a platinum catalyst, and NO is converted into NO₂ at the second rate in the second portion of the exhaust flow using a palladium catalyst.
 5. The method of claim 1, wherein particulate matter is removed from the first portion of the exhaust flow using a plurality of porous filter walls.
 6. A method of altering a composition of exhaust gas flow including particulate matter, hydrocarbons, and NO, the method comprising: directing a first portion of the exhaust flow through at least one first channel, wherein the at least one first channel is a wall-flow channel; directing a second portion of the exhaust flow through at least one second channel, wherein the at least one second channel is an open flow-through channel; converting NO into NO₂ in the first portion of the exhaust flow as the first portion of the exhaust flow passes through the at least one first channel; trapping the particulate matter in the first portion of the exhaust flow as the first portion of the exhaust flow passes through the at least one first channel; and passively regenerating the particulate matter in the second portion of exhaust flow as the second portion of exhaust flow passes from a first end of the at least one second flow channel through a second end of the at least one second flow channel.
 7. The method of claim 6, further comprising converting hydrocarbons in one or both of the first and second portions of the exhaust flow as the first and second portions of the exhaust flow pass through the at least one first channel and the at least one second channel, respectively.
 8. The method of claim 6, further comprising controlling the amount of particulate matter trapped and passively regenerated to provide a desired amount of particulate matter reduction in the exhaust flow.
 9. The method of claim 6, further comprising converting NO into NO₂ at a second rate in the second portion of the exhaust flow as the second portion of the exhaust flow passes through the at least one second channel; wherein the second rate is slower than the first rate, and the first and second rates are selected to provide a final ratio of NO to NO₂ of about 1:1 in the exhaust flow.
 10. The method of claim 6, wherein particulate matter is trapped in the at least one first channel using a plurality of porous filter walls.
 11. An exhaust filter, comprising: at least one first channel configured to convert NO in an exhaust flow to NO₂ at a first rate, the at least one first channel being a wall-flow channel configured to remove particulate matter from the exhaust flow; and at least one second channel configured to convert NO in an exhaust flow to NO₂ at a second rate, slower than the first rate, wherein the at least one second channel is an open flow-through channel.
 12. The exhaust filter of claim 11, further including a plurality of the at least one first and second channels, wherein a ratio of first channels to second channels is configured to provide a final ratio of NO to NO₂ of about 1:1.
 13. The exhaust filter of claim 11, wherein the at least one first channel is coated with a platinum catalyst and the at least one second channel is coated with a palladium catalyst.
 14. The exhaust filter of claim 11, wherein the at least one first channel includes a plurality of wall-flow channels and the at least one second channel includes a plurality of open flow-through channels.
 15. A method of adjusting a ratio of NO to NO₂ in a flow of exhaust gas, the method comprising: directing a first portion of the exhaust flow through at least one first channel, wherein the at least one first channel is a wall-flow channel; directing a second portion of the exhaust flow through at least one second channel, wherein the at least one second channel is an open flow-through channel; transforming NO into NO₂ in the first portion of the exhaust flow to yield a first ratio of NO to NO₂; transforming NO into NO₂ in the second portion of the exhaust flow to yield a second ratio of NO to NO₂; and combining the first and second portions of the exhaust flow as the first and second portions of the exhaust flow exit the at least one first and second flow channels to yield a final ratio of NO to NO₂ of about 1:1 in the combined exhaust flow.
 16. The method of claim 15, further comprising: trapping particulate matter in the at least one first channel to reduce the amount of particulate matter by a first percentage; and passively regenerating particulate matter in the exhaust flow in the at least one second channel to reduce the amount of particulate matter by a second percentage, wherein the first percentage is higher than the second percentage.
 17. The method of claim 15, wherein the at least one first channel includes a plurality of wall-flow channels and the at least one second channel includes a plurality of open flow-through channels.
 18. The method of claim 15, wherein NO is transformed into NO₂ in the first portion of the exhaust flow using a platinum catalyst and NO is transformed into NO₂ in the second portion of the exhaust flow using a palladium catalyst.
 19. The method of claim 15, wherein the first portion includes about 70% to 95% of the exhaust flow and the second portion includes about 5% to 30% of the exhaust flow.
 20. The method of claim 15, wherein NO is transformed into NO₂ in the first portion at a faster rate than in the second portion and wherein particulate matter is trapped in the first portion at a higher rate than in the second portion. 